Brakes "skip" as I press down. Whats goin on?

I have a 65 Dart in Ct and it's likely your brakes saw the same type salty winters in NYC/NY that promotes rapid rusting.
Selecting a master cylinder that bolts directly in starts by looking at your tire size.
If you see a 13 inch tire then it's likely you have 9 inch brake drums and the master cylinder should have the small bore around 15/16 or 1" diameter and be labeled for manual brakes. Note Rockauto doesn't tell you the bore diameter for the three you are looking at......... You would have to call them to get that info. Do not buy the larger bore master cylinders. You don't want to start this job unless you have an experienced person who knows how to bleed the air out of the brakes after the master cylinder is changed. Let us know your tire size. Do not order a master cylinder until it's verified you have a certain size brake drum.